Over the past decades there has been increasing pressure for ever more efficient and environmentally friendly aircraft to be designed. The use of waterborne aircraft could be a means of satisfying those requirements in the future. The aim of the PhD research program presented in this thesis was to develop the methodologies necessary for the preliminary design of large passenger seaplanes and evaluate the performance of such an aircraft compared to the current state of the art. The major technological and operational constraints in designing large waterborne aircraft were identified through an extensive feasibility study. A number of subject areas necessitating further investigation were also identified. To ensure that waterborne takeoff distance requirements are met, a novel initial sizing methodology was generated, relating the aircraft's thrust and lifting characteristics to the takeoff Balanced Field Length. To allow the design of a broad family of aircraft based on a predefined baseline configuration, the seaplane geometry was fully parameterized. The aerodynamic properties of the entire aircraft were determined using a vortex-lattice potential flow solver, written specifically for the configuration being investigated, combined with other commonly used empirical methods. Novel methodologies for estimating the hydrodynamic characteristics of a broad range of parametric hulls were developed using the wealth of experimental hydrodynamic test data available. These methods can be used not only to predict the resistance and trim characteristics of a seaplane throughout the entire takeoff and landing manoeuvre but also give an initial estimate of the attitudes where hydrodynamic instabilities may be encountered. The airborne and waterborne performance characteristics of each resulting aircraft design were estimated using the aforementioned methods. The resulting design synthesis has been integrated into a single algorithm, written in FORTRAN, intended to allow the easy and prompt analysis of any parametric variant of the baseline configuration.